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Evolution’s X factor: Shuffling up new species

By Bob Holmes

Darwin described the creation of new species as the “mystery of mysteries”. Could the solution be found in a single gene?

ALTHOUGH Charles Darwin titled his book On the Origin of Species, speciation was one thing he could not explain. He called it the “mystery of mysteries”, and even a century-and-a-half later the mechanism by which two groups of animals become genetically incompatible remains one of the greatest puzzles in biology. We understand how Darwin’s Galapagos finches could have evolved from a single species – different populations became isolated and gradually adapted to different environments until they were no longer able to reproduce with each other. However, speciation also occurs rapidly and without physical isolation of populations, which is far harder to explain. Yet, amazingly, a single gene could hold the key.

That gene is at the heart of a crucial but seemingly irrelevant process called genetic recombination. During the production of eggs and sperm, chromosomes pair up, cross over and swap segments of DNA, mixing the genes you inherited from your mother and father (see diagram). This shuffling of the genetic pack is the source our individual uniqueness. But no one suspected that recombination might also play a role in generating new species.

The breakthrough came with the discovery that the gene that controls genetic exchange, which goes by the prosaic name of PR-domain containing 9, or Prdm9, is also implicated in generating reproductive incompatibility between different members of the same species. Dubbed “the speciation gene”, if Prdm9‘s two roles turn out to be linked, it could be evolution’s missing X factor.